1. Field of the Invention
The present invention relates to an image processor such as a digital color copying machine having a function of enlarged copy and reduced copy.
2. Description of Related Art
A copying machine has a function to provide a copy with an enlarged or reduced image of a document. When a document image is enlarged, a number of image data read on a document is inflated, and the inflated data are interpolated. For example, in an apparatus explained in Japanese Patent laid open Publication No. 4-229765, a ratio of the inflated data is detected for each inflated data, and an appropriate filter is adopted according to the ratio. On the other hand, when a document image is reduced, data thinned out or a number of the data reduced according to a reduction ratio is used for reproduction. For example, when the number of the data is halved, the image data is thinned out every other pixel. Preferably, two memories are provided, so that image data can be written to one of the memories and read from the other thereof. Then, image data can be output continuously for image forming. Further, the image data is preferably stored in the memories after thinning-out, and the image data can be processed at a higher speed.
However, if a document image comprises dot images, a Moire pattern may happen when the reduction ratio is increased. Further, because the image data are thinned out in a simple way, the resolution of an image is deteriorated in proportion to the reduction ratio. Generally, in a monochromatic bi-level image, a ratio of white pixels is larger. Then, if image data is thinned out in such an image, a narrow line or a point may vanish in a reduced image, and this deteriorates image quality.